1. Field of the Invention
The present invention relates to a method of shutting down a water electrolysis apparatus which applies an electrolysis voltage between current collectors disposed on the respective sides of an electrolyte membrane thereby to electrolyze water to generate oxygen in an anode electrolysis chamber and hydrogen in a cathode electrolysis chamber having a pressure higher than a normal pressure.
2. Description of the Related Art
Solid polymer electrolyte fuel cells generate DC electric energy when anodes thereof are supplied with a fuel gas, i.e., a gas mainly containing hydrogen, e.g., a hydrogen gas, and cathodes thereof are supplied with an oxygen-containing gas, e.g., air.
Generally, water electrolysis apparatus are used to generate a hydrogen gas for use as a fuel gas for such solid polymer electrolyte fuel cells. The water electrolysis apparatus employ a solid polymer electrolyte membrane (ion exchange membrane) for decomposing water to generate hydrogen (and oxygen). Electrode catalyst layers are disposed on the respective sides of the solid polymer electrolyte membrane, making up a membrane electrode assembly. Current collectors are disposed on the respective sides of the membrane electrode assembly, making up a unit. The unit is essentially similar in structure to the fuel cells described above.
A plurality of such units are stacked, and a voltage is applied across the stack while water is supplied to the current collectors on the anode side. On the anodes of the membrane electrode assemblies, the water is decomposed to produce hydrogen ions (protons). The hydrogen ions move through the solid polymer electrolyte membranes to the cathodes, where the hydrogen ions combine with electrons to generate hydrogen. On the anodes, oxygen generated together with hydrogen ion is discharged with excess water from the units.
As such a water electrolysis apparatus, there has been used a high-pressure hydrogen manufacturing apparatus for generating hydrogen under a high pressure which is generally of 1 MPa or higher. For example, a high-pressure hydrogen manufacturing apparatus disclosed in Japanese Laid-Open Patent Publication No. 2006-070322 comprises a solid polymer membrane, cathode current collectors disposed on the respectively opposite sides of the solid polymer membrane in confronting relation to each other, anode current collectors, separators stacked on the current collectors, and flow fields defined in the separators, to which the electrode feeders are exposed. When water is supplied to the flow fields defined in the anode separators and the current collectors are energized, the water is electrolyzed to produce a hydrogen gas under high pressure in the flow fields defined in the cathode separators. The disclosed high-pressure hydrogen manufacturing apparatus includes pressing means for pressing the cathode current collectors into intimate contact with the solid polymer membrane.
When a pressure on the cathode is high, the pressing means presses the cathode current collectors into intimate contact with the solid polymer membrane. Therefore, no clearance is created between the cathode current collectors and the solid polymer membrane, preventing the contact resistance from increasing.
In the above high-pressure hydrogen manufacturing apparatus, the flow fields defined in the cathode separators are filled with the high-pressure hydrogen, and on the other side of the solid polymer membrane, water and oxygen under a normal pressure are present in the flow fields defined in the anode separators. When the high-pressure hydrogen manufacturing apparatus is to be shut down, i.e., when the high-pressure hydrogen manufacturing apparatus is to stop supplying the generated hydrogen, it is necessary to eliminate the pressure difference across the solid polymer membrane in order to protect the solid polymer membrane.
For shutting down the high-pressure hydrogen manufacturing apparatus, it is customary to de-energize the current collectors to stop the water electrolyzing process, and then the pressure of the hydrogen filling the flow fields defined in the cathode separators is forcibly reduced to a level near the normal pressure.
If the pressure of the hydrogen is reduced too rapidly, the solid polymer membrane and seals tend to be unduly damaged. Therefore, the pressure of the hydrogen needs to be reduced slowly. As a consequence, it takes a considerable period of time until the pressure of the hydrogen filling the flow fields defined in the cathode separators reaches the normal pressure after the water electrolyzing process is stopped. During such a long period of time, the hydrogen is liable to pass from the cathode to the anode, causing the anode catalysts to be reduced by the hydrogen to result in deterioration in the water electrolyzing capability.